Question

Describe the operation of a multiple-access technique in cellular communication.

Answer 1

A multiple-access technique in cellular communication is a method that allows multiple users to share the limited radio frequency spectrum efficiently and concurrently within a cellular network. This is crucial for enabling a large number of users to communicate simultaneously in a cellular system without causing excessive interference or degradation in the quality of communication. One of the most commonly used multiple-access techniques is known as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA).

Frequency Division Multiple Access (FDMA):
In FDMA, the available frequency spectrum is divided into multiple non-overlapping frequency bands or channels. Each channel is allocated to a specific user or communication session. Users transmit and receive their data using separate frequency bands, which helps prevent interference between different users. FDMA is often used in analog cellular systems and some modern digital systems like 2G GSM.

Time Division Multiple Access (TDMA):
TDMA divides the available time into discrete time slots. Each user is allocated one or more time slots within a predefined time frame. Users take turns transmitting and receiving within their assigned time slots. This method efficiently utilizes the available bandwidth because multiple users share the same frequency spectrum without overlapping in time. TDMA is used in digital cellular systems such as 2G GSM and 3G UMTS.

Code Division Multiple Access (CDMA):
CDMA is a more advanced multiple-access technique where all users in a cell use the entire available frequency spectrum simultaneously. Instead of dividing by frequency or time, CDMA assigns a unique code to each user's signal. These codes are designed to be orthogonal, meaning they have minimal interference with each other. Each user's data is spread across the entire spectrum using their unique code, and the receiver is able to de-spread the signal to recover the original data. CDMA is used in 3G (CDMA2000) and 4G (LTE) cellular networks.

These multiple-access techniques are integral to the operation of cellular communication systems as they allow for efficient use of the limited radio frequency spectrum, which is a valuable and finite resource. The choice of which technique to use can depend on factors such as the technology being employed, the specific cellular standards in use, and the trade-offs between spectral efficiency, capacity, and system complexity.